Bind outbound connections to a local address/interface via local_addr (#834)

[quinnj]

Closes #834.

Adds a local_addr keyword to Transport and Client that binds outbound connections to a specific source IP — and therefore the outgoing interface — modeled on Go's net.Dialer.LocalAddr (and curl --interface). Useful on multi-homed hosts and for separating traffic by interface.

# every request from this client leaves via 192.0.2.10
client = HTTP.Client(local_addr = "192.0.2.10")
HTTP.get("http://example.com"; client = client)

# canonical home — local_addr is a connection-pool property:
client = HTTP.Client(transport = HTTP.Transport(local_addr = "192.0.2.10"))

Design

Reseau's HostResolver/TCP.connect already accept a local bind endpoint (Reseau = "1.3" has it); this just threads it through HTTP's transport. Two deliberate choices, both following Go:

• Per-Transport/Client, not per-request. local_addr is a connection-identity property: a connection bound to source A cannot serve a request that wants source B. Go puts it on the Dialer, not the request, precisely so it folds into the pool. Each bound Client keeps its own pool, so binding needs no pool-key changes and can't cause cross-binding connection reuse.
• IP, not interface name. Accepts an IP-literal String (kernel-chosen ephemeral source port) or a Reseau.TCP.SocketAddrV4/SocketAddrV6 for a fixed source port. Interface names (curl --interface eth0) are intentionally not accepted — like Go, the address is an IP, keeping name resolution out of the transport.

Binding to an address not assigned to a local interface fails fast at bind(). Passing local_addr alongside an explicit transport is rejected as ambiguous.

Tests

http_client_transport_tests.jl: the normalizer (string→endpoint, IPv6, passthrough, invalid→ArgumentError), client- and transport-level binding round-trips, the ambiguity guard, and that an unassigned source address fails — the deterministic proof the bind is actually applied rather than ignored. Documents the knob in the client guide.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.16%. Comparing base (427b525) to head (33685f2).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1307      +/-   ##
==========================================
- Coverage   87.16%   87.16%   -0.01%     
==========================================
  Files          28       29       +1     
  Lines       11042    11289     +247     
==========================================
+ Hits         9625     9840     +215     
- Misses       1417     1449      +32     

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